Israel and the United States have threatened possible military action if diplomacy and increasingly tough trade and energy sanctions fail to make Iran curb its nuclear program.

Talks between the UN nuclear watchdog and Iran about Tehran's nuclear program failed to clinch an accord and no date has been set for more meetings, a senior UN official said on Wednesday.

"We had intensive discussions today but did not finalize the structured approach document that has been under negotiation for a year and a half now," Herman Nackaerts, deputy director general of the International Atomic Energy Agency (IAEA), told reporters after meeting Iranian officials in Vienna.

"Our commitment to continue dialog is unwavering. However, we must recognize that our best efforts have not been successful so far. So we will continue to try and complete this process."

Any movement in the decade-old standoff will probably have to wait until after Iranians vote on June 14 for a successor to President Mahmoud Ahmadinejad, analysts and diplomats say.

Even though it is Supreme Leader Ayatollah Ali Khamenei who decides Iran's nuclear policy, the conservative leadership may want to tread cautiously ahead of a poll in which loyalists will be challenged by two major independents.

With the election coming up, "the Iranians will do everything to keep everything stable," one Western envoy said.

Israel and the United States have threatened possible military action if diplomacy and increasingly tough trade and energy sanctions fail to make Iran curb its nuclear program.

Tehran says its nuclear activity has only peaceful purposes and that it is Israel, widely believed to be the Middle East's only nuclear-armed power, that threatens peace and stability in the region.The IAEA has been trying for more than a year to coax Iran into letting it resume an inquiry into what the UN watchdog calls the "possible military dimensions" of its nuclear work.

Wednesday's talks in Vienna will be the 10th round of negotiations between the two sides since early 2012, so far without a framework agreement that would give the IAEA the access it wants to sites, officials and documents.

Iran's IAEA envoy, Ali Asghar Soltanieh, said this week he expected progress to be made in the discussions. But Western diplomats voiced pessimism.

The Istanbul meeting between EU foreign policy chief Catherine Ashton, who represents six world powers, and Iran's chief nuclear negotiator Saeed Jalili follows a failed round of big-power diplomacy in Kazakhstan in early April.

The gap is wide: the powers want Iran to suspend its most sensitive nuclear activity. Iran wants them to recognize its "right" to enrich uranium - which can have both civilian and military purposes - and to end tough economic sanctions.